The invention relates to a method of manufacturing a separation column which comprises a glass capillary which is coated with a polymer film on the inside.
The invention further relates to a separation column comprising a glass capillary having an internal diameter of maximally 10 .mu.m and, which is coated with a polymer film on the inside.
Such separation columns are used in liquid chromatography and in separation techniques which are based on the movement of particles in an electric field, such as capillary zone electrophoresis and electroendosmotic chromatography.
Liquid chromatography is a known method of separating, detecting and analysing mixtures of chemical compounds and is particularly suitable for the analysis of non-volatile or thermally unstable solid or liquid compounds such as many organic chemical compounds. The sample to be analysed is dissolved in a suitable solvent, the so-called mobile phase, and pressed through a column with the same or another suitable solvent at an increased pressure. Columns which are often used are the so-called packed columns in which a suitable absorbent in the form of porous grains is used as the stationary phase. Customarily, the grains have been subjected to a surface treatment and the thin surface layer formed serves as the stationary phase. By using a capillary, which is coated on the inside with a suitable stationary phase, instead of a packed column it is possible, in principle, to improve the separatory power of the column as well as the rate of separation. Such capillary columns are termed in literature OTLC-(Open tubular liquid chromatography) columns. It can be derived theoretically that a capillary column is superior to a packed column when the internal diameter of the capillary is 5-10 .mu.m and the thickness of the stationary phase on the inside of the capillary is 0.3-1 .mu.m. Owing to the difference in affinity for the stationary phase, the chemical compounds in the sample to be analysed exhibit different retention times in the chromatographic column. In this manner, the sample is separated into the individual compounds present. Detection and quantitative and/or qualitative determination of the compounds takes place at the end of the column.
A method of the type mentioned in the opening paragraph is described in an article by O. van Berkel et al. in Chromatographia 24 739-744 (1987), the contents of which are hereby incorporated by reference. In the method described in this article, a solution of a dimethylsiloxane polymer and a thermal initiator in pentane is pressed into the column. The pentane is evaporated at a reduced pressure after which crosslinking of the polymer film formed takes place at an increased temperature.
A disadvantage of the known method is the small layer thickness of the resultant polymer film in capillaries having a small internal diameter. A layer thickness of 0.045 .mu.m is disclosed at an internal capillary diameter of 5 .mu.m. In this case, a criterion is the phase ratio .beta., which is equal to ##EQU1## In this formula, V.sub.s is the volume of the stationary phase; V.sub.m is the volume of the mobile phase; d is the thickness of the polymer film (=stationary phase) and r is the radius of the column after the polymer film is applied. In the example given, .beta.=0.037. The thickness of the thickest polymer film mentioned is 0.70 .mu.m at a capillary diameter of 25.1 .mu.m, which corresponds to a value of .beta. of 0.122. In general, thicker polymer layers can be obtained by increasing the concentration of the polymer in the pentane solution. However, this leads to solutions having a high viscosity so that the capillary can hardly be filled with the polymer solution. The known method is unsuitable for internally coating thin capillaries with a relatively thick polymer film. The possibility of manufacturing thick polymer films is of essential importance to the usability of the column. The maximum load (capacity) of the column increases as the thickness increases, which results in a better detection of the components.